Buff for Garen Ultimate.
Well, his ultimate is an execution, and therefore when he kills a enemy with it, it should lower the cooldown by around 10-20 seconds. Scaling with +10 seconds reduction of it each time it's ranked up. If Darius's ultimate can have a cooldown reset at rank 3, and be able to cast his ultimate within a frame of 20 seconds after killing someone with it, I feel that Garen should have the similar power he has with his main execution tool.